selftests: timers: clocksource-switch: add command line switch to skip sanity check

The sanity check takes a while. If you do repeated checks when
debugging, this is time consuming. Add a parameter to skip it.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Acked-by: John Stultz <jstultz@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>